Protein tyrosine phosphatase PTPN22 negatively modulates platelet function and thrombus formation.

Protein tyrosine phosphatase nonreceptor type 22 (PTPN22) is a protein tyrosine phosphatase that negatively regulates T-cell signaling. However, whether it is expressed and functions in platelets remains unknown. Here we investigated the expression and role of PTPN22 in platelet function. We reported PTPN22 expression in both human and mouse platelets. Using PTPN22-/- mice, we showed that PTPN22 deficiency significantly shortened tail-bleeding time and accelerated arterial thrombus formation without affecting venous thrombosis and the coagulation factors VIII and IX. Consistently, PTPN22-deficient platelets exhibited enhanced platelet aggregation, granule secretion, calcium mobilization, lamellipodia formation, spreading, and clot retraction. Quantitative phosphoproteomic analysis revealed the significant difference of phosphodiesterase 5A (PDE5A) phosphorylation in PTPN22-deficient platelets compared with wild-type platelets after collagen-related peptide stimulation, which was confirmed by increased PDE5A phosphorylation (Ser92) in collagen-related peptide-treated PTPN22-deficient platelets, concomitant with reduced level and vasodilator-stimulated phosphoprotein phosphorylation (Ser157/239). In addition, PTPN22 interacted with phosphorylated PDE5A (Ser92) and dephosphorylated it in activated platelets. Moreover, purified PTPN22 but not the mutant form (C227S) possesses intrinsic serine phosphatase activity. Furthermore, inhibition of PTPN22 enhanced human platelet aggregation, spreading, clot retraction, and increased PDE5A phosphorylation (Ser92). In conclusion, our study shows a novel role of PTPN22 in platelet function and arterial thrombosis, identifying new potential targets for future prevention of thrombotic or cardiovascular diseases.

The Selective SIRT3 Inhibitor 3-TYP Represses Primary Myeloma Growth by Reducing c-Myc Stability.

Multiple myeloma is a hematological cancer that can be treated but remains incurable. With the advancement of science and technology, more drugs have been developed for myeloma chemotherapy that greatly improve the quality of life of patients. However, relapse remains a serious problem puzzling patients and doctors. Thus, developing more highly active and specific inhibitors is urgent for myeloma-targeted therapy. In this study, we identified the SIRT3 inhibitor 3-TYP (3-(1H-1,2,3-triazol-4-yl) pyridine) after screening a histone modification compound library, which showed high cytotoxicity and induced DNA damage in myeloma cells. Furthermore, the inhibitory effect of 3-TYP in our xenograft tumor studies also confirmed that compound 3-TYP could inhibit primary myeloma growth by reducing c-Myc protein stability by decreasing c-Myc Ser62 phosphorylation levels. Taken together, the results of our study identified 3-TYP as a novel c-Myc inhibitor, which could be a potential chemotherapeutic agent to target multiple myeloma.

A novel 2-iminobenzimidazole compound, XYA1353, displays in vitro and in vivo anti-myeloma activity via targeting NF-κB signaling.

Multiple myeloma (MM) is an accumulated disease of malignant plasma cells, which is still incurably owing to therapeutic resistance and disease relapse. Herein, we synthesized a novel 2-iminobenzimidazole compound, XYA1353, showing a potent anti-myeloma activity both in vitro and in vivo. Compound XYA1353 dose-dependently promoted MM cell apoptosis via activating caspase-dependent endogenous pathways. Moreover, compound XYA1353 could enhance bortezomib (BTZ)-mediated DNA damage via elevating γH2AX expression levels. Notably, compound XYA1353 interacted synergistically with BTZ and overcame drug resistance. RNA sequencing analysis and experiments confirmed that compound XYA1353 inhibited primary tumor growth and myeloma distal infiltration by disturbing canonical NF-κB signaling pathway via decreasing expression of P65/P50 and p-IκBα phosphorylation level. Due to its importance in regulating MM progression, compound XYA1353 alone or combined with BTZ may potentially exert therapeutic effects on multiple myeloma by suppressing canonical NF-κB signaling.

Novel c-Myc G4 stabilizer EP12 promotes myeloma cytotoxicity by disturbing NF-κB signaling.

Multiple myeloma (MM) is a B-cell malignancy characterized by the excessive proliferation of bone marrow plasma cells and the production of abnormal immunoglobulins. Despite advances in therapeutic strategies, it remains an incurable disease. Recently, innovative anticancer drugs have been developed and approved, leading to improvements in MM therapy; however, drug resistance continues to be a major obstacle that results in treatment failure. Therefore, the development of novel agents is imperative to achieve superior therapeutic outcomes for relapsed/refractory multiple myeloma (MM) patients. Previously, we identified EP12 as a c-Myc G4 stabilizer that could induce cytotoxicity in MM cells in vitro. However, further investigation is required to elucidate the underlying molecular mechanisms and anti-MM activity of EP12 in vivo. In this study, we have discovered that the compound EP12 effectively inhibits primary myeloma growth in vivo by destabilizing c-Myc and disrupting the canonical nuclear factor-κB (NF-κB) signaling pathway. Overall, our findings suggest that EP12, as a potent c-Myc inhibitor, holds great promise as a therapeutic agent for MM.

Novel dual-targeting c-Myc inhibitor D347-2761 represses myeloma growth via blocking c-Myc/Max heterodimerization and disturbing its stability.

BACKGROUND: Transcription factor c-Myc plays a critical role in various physiological and pathological events. c-Myc gene rearrangement is closely associated with multiple myeloma (MM) progression and drug resistance. Thereby, targeting c-Myc is expected to be a useful therapeutic strategy for hematological disease, especially in MM. METHODS: Molecular docking-based virtual screening and dual-luciferase reporter gene assay were used to identify novel c-Myc inhibitors. Cell viability and flow cytometry were performed for evaluating myeloma cytotoxicity. Western blot, immunofluorescence, immunoprecipitation, GST pull down and Electrophoretic Mobility Shift Assay were performed for protein expression and interaction between c-Myc and Max. c-Myc downstream targets were measured by Q-PCR and Chromatin immunoprecipitation methods. Animal experiments were used to detect myeloma xenograft and infiltration in vivo. RESULTS: We successfully identified a novel c-Myc inhibitor D347-2761, which hindered the formation of c-Myc/Max heterodimer and disturbed c-Myc protein stability simultaneously. Compound D347-2761 dose-and time-dependently inhibited myeloma cell proliferation and induced apoptosis. Dual knockout Bak/Bax partially restored D347-2761-mediated cell death. Additionally, compound D347-2761 could, in combination with bortezomib (BTZ), enhance MM cell DNA damage and overcome BTZ drug resistance. Our in vivo studies also showed that compound D347-2761 repressed myeloma growth and distal infiltration by downregulating c-Myc expression. Mechanistically, novel dual-targeting c-Myc inhibitor D347-2761 promoted c-Myc protein degradation via stimulating c-Myc Thr58 phosphorylation levels, which ultimately led to transcriptional repression of CDK4 promoter activity. CONCLUSIONS: We identified a novel dual-targeting c-Myc small molecular inhibitor D347-2761. And this study may provide a solid foundation for developing a novel therapeutic agent targeting c-Myc. Video Abstract.

High expression of miR-25 predicts favorable chemotherapy outcome in patients with acute myeloid leukemia.

BACKGROUND: Acute myeloid leukemia (AML) pertains to a hematologic malignancy with heterogeneous therapeutic responses. Improvements in risk stratification in AML patients are warranted. MicroRNAs have been associated with the pathogenesis of AML. METHODS: To examine the prognostic value of miR-25, 162 cases with de novo AML were classified into two groups according to different treatment regimens. RESULTS: In the chemotherapy group, cases with upregulated miR-25 expression showed relatively longer overall survival (OS; P = 0.0086) and event-free survival (EFS; P = 0.019). Multivariable analyses revealed that miR-25 upregulation is an independent predictor for extended OS (HR = 0.556, P = 0.015) and EFS (HR = 0.598, P = 0.03). In addition, allogeneic hematopoietic stem cell transplantation (allo-HSCT) circumvented the poor prognosis that was related to miR-25 downregulation with chemotherapy. The expression level pattern of miR-25 coincided with AML differentiation and proliferation, which included HOXA and HOXB cluster members, as well as the HOX cofactor MEIS1. The MYH9 gene was identified as a direct target of miR-25. CONCLUSIONS: The miR-25 levels are correlated with prognosis in AML independently of other powerful molecular markers. The expression of miR-25 may contribute to the selection of the optimal treatment regimen between chemotherapy and allo-HCST for AML patients.

Anti-Proliferative Activity of HPOB against Multiple Myeloma Cells via p21 Transcriptional Activation.

Histone acetylation or deacetylation is closely associated with the progression of multiple myeloma (MM). Currently, many histone deacetylase (HDAC) inhibitors have been approved for being used in clinical trials, but theirtherapeutic effectsarestill not ideal. As a novel HDAC inhibitor, hydroxamicacid-based small-moleculeN-hydroxy-4-(2-[(2-hydroxyethyl)(phenyl)amino]-2-oxoethyl)benzamide (HPOB)'s possible roles in MM have not been studied. In this present study, the effect of HPOB as a potential anti-tumor agent in preventingproliferation and inducing apoptosis of MM cells had been investigated in detail. Our results showed that HPOB decreased the survival of MM cells in dose- and time-dependent manner. In addition, HPOB caused the accumulation of MM cells in G1 phase compared with the dimethylsulfoxide (DMSO) control group. Interestingly, we found that HPOB could overcome bortezomib (BTZ) resistance inMM cells and combining HPOB with BTZ could further sensitize MM cells. Certainly, our data illuminated that HPOB-mediated cell death occurs via transcriptional activation of p21, which was associated with an elevated level of global histone 3 acetylation (H3Ac) modification. Therefore, HPOB could be a potential candidate for MM treatment and the combination of HPOB and bortezomibcould bea possible therapeutic strategy for relapsed and refractory MM.

Pretreatment neutrophil/lymphocyte ratio but not platelet/lymphocyte ratio has a prognostic impact in multiple myeloma.

BACKGROUND: We evaluated the prognostic significance of neutrophil/lymphocyte ratio (NLR) and platelet/lymphocyte ratio (PLR) in patients with multiple myeloma (MM). METHODS: In total, we retrospectively analyzed 315 newly diagnosed MM patients and calculated NLR and PLR from the complete blood count of the untreated patients. We further assessed the role of pretreatment NLR and PLR on overall survival (OS) and progression-free survival (PFS). RESULTS: Multiple myeloma patients with high NLR (≥2) experienced shorter OS (P=.02) and PFS (P=.01) compared with patients with low NLR (<2). Furthermore, among the patients with conventional chemotherapy, elderly patients, or patients with advanced stages, high NLR (≥2) was found to have a negative prognostic impact on OS and PFS. In the multivariate Cox analysis, we confirmed that the NLR was an independent prognostic factor for both OS and PFS (P=.000). But the differences in OS or PFS by PLR were not found in MM patients. CONCLUSIONS: Our study suggests that NLR not PLR can be acted as an independent prognostic factor for analyzing the clinical outcome of MM patients.

Suppression of RAD21 Induces Senescence of MDA-MB-231 Human Breast Cancer Cells Through RB1 Pathway Activation Via c-Myc Downregulation.

Cellular senescence impedes cancer progression by limiting uncontrolled cell proliferation. To identify new genetic events controlling senescence, we performed a small interfering RNA screening human cancer cells and identified a number of targets potentially involved in senescence of MDA-MB-231 human breast cancer cells. Importantly, we showed that knockdown of RAD21 resulted in the appearance of several senescent markers, including enhanced senescence-associated ß-galactosidase activity and heterochromatin focus formation, as well as elevated p21 protein levels and RB1 pathway activation. Further biochemical analyses revealed that RAD21 knockdown led to the downregulation of c-Myc and its targets, including CDK4, a negative regulator of RB1, and blockedRB1 phosphorylation (pRB1), and the RB1-mediated transcriptional repression of E2F. Moreover, c-Myc downregulation was partially mediated by proteasome-dependent degradation within promyelocytic leukemia (PML) nuclear bodies, which were found to be highly abundant during RAD21 knockdown-induced senescence. Exogenous c-Myc reconstitution rescued cells from RAD21 silencing-induced senescence. Altogether, data arising from this study implicate a novel function of RAD21 in cellular senescence in MDA-MB-231 cells that is mainly dependent onRB1 pathway activation via c-Myc downregulation.

Anti-G Protein-Coupled Receptor, Class C Group 5 Member D Chimeric Antigen Receptor T Cells in Patients With Relapsed or Refractory Multiple Myeloma: A Single-Arm, Phase â ¡ Trial.

PURPOSE: G protein-coupled receptor, class C group 5 member D (GPRC5D) is considered to be a promising surface target for multiple myeloma (MM) immunotherapy. Here, we report the efficacy and safety of anti-GPRC5D chimeric antigen receptor (CAR) T cells in patients with relapsed or refractory (R/R) MM. METHODS: This phase Ⅱ, single-arm study enrolled patients (18-70 years) with R/R MM. Lymphodepletion was performed before patients received 2 × 106/kg anti-GPRC5D CAR T cells. The primary end point was the proportion of patients who achieved an overall response. Safety was also evaluated in eligible patients. RESULTS: From September 1, 2021, to March 23, 2022, 33 patients were infused with anti-GPRC5D CAR T cells. At a median follow-up of 5.2 months (range, 3.2-8.9), the overall response rate was 91% (95% CI, 76 to 98; 30 of 33 patients), including 11 (33%) stringent complete responses, 10 (30%) complete responses, four (12%) very good partial responses, and five (15%) partial responses. Partial responses or better were observed in nine (100%) of nine patients with previous anti-B-cell maturation antigen (BCMA) CAR T-cell therapy, including two patients who had received repeated anti-BCMA CAR T-cell infusions with no responses at the last time. Grade 3 or higher hematologic toxicities were neutropenia (33 [100%]), anemia (17 [52%]), and thrombocytopenia (15 [45%]). Cytokine release syndrome occurred in 25 (76%) of 33 patients (all were grade 1 or 2), and neurotoxicities in three patients (one grade 2 and one grade 3 ICANSs and one grade 3 headache). CONCLUSION: Anti-GPRC5D CAR T-cell therapy showed an encouraging clinical efficacy and manageable safety profile in patients with R/R MM. For patients with MM that progressed after anti-BCMA CAR T-cell therapy or that is refractory to anti-BCMA CAR T cell, anti-GPRC5D CAR T-cell therapy might be a potential alternative option.

TSA-induced JMJD2B downregulation is associated with cyclin B1-dependent survivin degradation and apoptosis in LNCap cells.

Histone deacetylase (HDAC) inhibitors are emerging as a novel class of anti-tumor agents and have manifested the ability to induce apoptosis of cancer cells, and a significant number of genes have been identified as potential effectors responsible for HDAC inhibitor-induced apoptosis. However, the mechanistic actions of these HDAC inhibitors in this process remain largely undefined. We here report that the treatment of LNCap prostate cancer cells with HDAC inhibitor trichostatin A (TSA) resulted in downregulation of the Jumonji domain-containing protein 2B (JMJD2B). We also found that the TSA-mediated decrease in survivin expression in LNCap cells was partly attributable to downregulation of JMJD2B expression. This effect was attributable to the promoted degradation of survivin protein through inhibition of Cyclin B1/Cdc2 complex-mediated survivin Thr34 phosphorylation. Consequently, knockdown of JMJD2B enhanced TSA-induced apoptosis by regulating the Cyclin B1-dependent survivin degradation to potentiate the apoptosis pathways.

Identification of Potential Prognostic Biomarker for Predicting Survival in Multiple Myeloma Using Bioinformatics Analysis and Experiments.

Multiple myeloma (MM) is a malignant disease of plasma cells, which remains incurable because of its unclear mechanism and drug resistance. Herein, we aimed to explore new biomarkers and therapeutic targets in MM. After screening differentially expressed genes (DEGs) in GSE6477 and GSE13591 dataset, we performed Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses of DEGs using DAVID online database. The results indicated that the downregulated DEGs were mainly enriched in the immune-associated biological process. The protein-protein interaction network was constructed by STRING database, on which we performed module analysis and identified key genes. Gene set enrichment analysis (GSEA) and Kaplan-Meier analysis showed that RRM2 could be a novel biomarker in MM diagnosis. We further confirmed that novel RRM2 inhibitor osalmid inhibited MM cell proliferation and triggered cell cycle S phase arrest. Targeting RRM2 was expected to develop new therapeutic strategies for malignant MM.

Design, Synthesis and Pharmacological Evaluation of Naphthofuran Derivatives as Potent SIRT1 Activators.

Diabetic nephropathy (DN) is one of the most important medical complications in diabetic patients, which is an essential cause of end-stage renal disease in diabetic patients and still lacks effective medicines. Silent information regulator 1 (SIRT1) is closely related to the occurrence and development of DN. Activation of SIRT1 could significantly improve the symptoms of DN, while the activities of SIRT1 activators need to be further improved. Based on the crystal structure of SIRT1, structure and ligand-based approaches were carried out, and a lead compound 4,456-0661 (renamed as M1) was identified. Moreover, seven M1 analogues (6a-6g) were designed using a structure-based drug design strategy followed by bioactivity evaluation with SRTR2104 used as positive drugs. Among the target molecules, compounds M1, 6b, and 6d were proved to be potent SIRT1 activators, the activities of which are comparable to SRT2104. More importantly, compounds M1, 6b, and 6d could resist high glucose-induced apoptosis of HK-2 cells by activating SIRT1 and deacetylation of p53. Apart from the beneficial effect on apoptosis of DN, these compounds also alleviated high glucose stimulating inflammation response in HK-2 cells through SIRT1/NF-κB (p65) pathway. Consequently, M1, 6b, and 6d could be promising drug candidates for SIRT1 related diseases.

Integrating molecular dynamics simulation and molecular mechanics/generalized Born surface area calculation into pharmacophore modeling: a case study on the proviral integration site for Moloney murine leukemia virus (Pim)-1 kinase inhibitors.

Communicated by Ramaswamy H. Sarma.

Identification of a Novel c-Myc Inhibitor 7594-0037 by Structure-Based Virtual Screening and Investigation of Its Anti-Cancer Effect on Multiple Myeloma.

INTRODUCTION: Multiple myeloma (MM) is an extremely malignant and incurable hematological cancer. Increased expression of the c-Myc oncoprotein is closely associated with shorter overall survival of MM patients, implying that c-Myc is a potential therapeutic target. MAIN METHODS: We identified a potential c-Myc inhibitor 7594-0037 by structure-based virtual screening from the ChemDiv database. CCK8 assay and flow cytometry were used to detect MM cell viability, cell cycle and apoptosis. Q-PCR and Western blot were used to measure corresponding mRNA and protein expression levels. Protein stability assay measured the stability of c-Myc. RESULTS: Compound 7594-0037 exhibited stronger anti-proliferative activity against MM cells, and induced MM cell cycle G2 phase arrest and apoptosis. More importantly, compound 7594-0037 overcame myeloma resistance to bortezomib and exhibited a synergistic effect with bortezomib, resulting in increased MM cell death. The mechanism consists of compound 7594-0037 facilitating c-Myc protein degradation via decreasing the c-Myc S62 phosphorylation levels mediated by PIM1 kinase. Molecular dynamics simulation with the c-Myc/7594-0037 complex showed that compound 7594-0037 bound tightly to the N-terminus of c-Myc, and blocked the binding interaction of the two termini of c-Myc, which resulted in c-Myc entering into an unstable state. CONCLUSION: Overall, our study provides preliminary data for compound 7594-0037, which can be used as a novel c-Myc inhibitor and is a potential candidate therapeutic drug for multiple myeloma.

The selective HDAC6 inhibitor Nexturastat A induces apoptosis, overcomes drug resistance and inhibits tumor growth in multiple myeloma.

Multiple myeloma (MM) is a hematological malignancy of plasma cells that produce a monoclonal immunoglobulin protein. Despite significant advances in the treatment of MM, challenges such as resistance to therapy remain. Currently, inhibition of histone deacetylases (HDACs) is emerging as a potential method for treating cancers. Numerous HDAC inhibitors are being studied for the use in monotherapy or in conjunction with other agents for MM. In the present study, we investigated the anti-myeloma effect of Nexturastat A (NexA), a novel selective HDAC6 inhibitor. We found that NexA impaired MM cells viability in a dose- and time-dependent manner. NexA also provoked a cell cycle arrest at the G1 phase in MM cells. Furthermore, NexA promoted apoptosis of MM cells via transcriptional activation of the p21 promoter, which may through its ability to up-regulate the H3Ac and H4Ac levels. Additionally, NexA could overcome bortezomib (BTZ) resistance in MM cells, and NexA in combination with BTZ had stronger efficacy. We also confirmed that NexA inhibited tumor growth in murine xenograft models of MM. These interesting findings provided the rationale for the future advancement of this novel HDAC6 inhibitor as a potential therapeutic anti-myeloma agent.

Anticancer effect of histone deacetylase inhibitor scriptaid as a single agent for hepatocellular carcinoma.

Recurrence is one of the major causes of poor prognosis for patients with hepatocellular carcinoma (HCC), and drug resistance is closely associated with disease recurrence. Histone deacetylase (HDAC) inhibitor scriptaid functions as an anticancer agent in many different types of tumors, but its possible roles in HCC progression have not been explored to date. Herein, we show that HDAC inhibitor scriptaid decreases HCC cell proliferation and induces cell cycle G2/M-phase arrest in a dose-dependent manner. Furthermore, scriptaid triggered HCC cell death via transcriptional activation of p21 and subsequent elevated global H3Ac levels. Importantly, we found that scriptaid showed robust antitumor activity against HCC. Thus, our findings indicate that HDAC inhibitor scriptaid could be an important potential candidate for treatment of HCC patients.

Lithium chloride inhibits cell survival, overcomes drug resistance, and triggers apoptosis in multiple myeloma via activation of the Wnt/ß-catenin pathway.

Multiple myeloma (MM) is an extremely malignant plasma cell disease, which is still incurable due to its drug resistance. Lithium chloride (LiCl) functions in many pathological processes, including bipolar disorder, acute brain injuries, and chronic neurodegenerative diseases, but its antagonistic role in MM progression has not been reported thus far. In this study, we found that LiCl inhibited MM cell proliferation and induced MM cell cycle G2/M phase arrest in a dose-dependent manner. Moreover, LiCl overcomes bortezomib (BTZ)-mediated resistance in MM cells and induces apoptosis in BTZ-resistant cells. Our data preliminarily indicate that LiCl induces MM cell apoptosis via activating the Wnt/ß-catenin signaling pathway. Overall, our results define LiCl as an inducer of MM cell apoptosis and unveil a crosstalk between BTZ and LiCl in facilitating cell apoptosis.

Identification of a novel c-Myc inhibitor with antitumor effects on multiple myeloma cells.

Increasing evidence shows that c-Myc oncoprotein is tightly associated with multiple myeloma (MM) progression. Herein, we identified compound 7594-0035, which is a novel inhibitor that specifically targets c-Myc. It was identified from the ChemDiv compound database by molecular docking-based, high-throughput virtual screening. Compound 7594-0035 inhibited MM cell proliferation in vitro, induced cell cycle G2-phase arrest, and triggered MM cell death by disturbing the stability of c-Myc protein. Additionally, we also found that compound 7594-0035 overcame bortezomib (BTZ) drug resistance and increased the killing effect on MM cells in combination with BTZ. The severe combined immune deficiency (SCID) mouse xenograft model revealed that compound 7594-0035 partially decreased the primary tumor growth of Roswell Park Memorial Institute (RPMI)-8226 cells in vivo The novel small molecular compound 7594-0035 described in the present study that targets c-Myc protein is likely to be a promising therapeutic agent for relapsed/refractory MM.

[Effects of HDAC Inhibitor Scriptaid on IM9 Cell Line and Its Mechanism].

OBJECTIVE: To study the effect of HDAC inhibitor Scriptaid on multiple myeloma IM9 cells and preliminarily clarify the mechanism of Scriptaid-induced cell apoptosis. METHODS: The cell viability, cell cycle and cell apoptosis were measured by CCK8 assay and flow cytometry respectively, the relative target gene expression levels were detected by RT-PCR, the effect of Scriptaid on p21 promoter activity was detected by using luciferase reporter assay. RESULTS: Scriptaid inhibited IM9 cell viability in a dose-dependent manner. Scriptaid induced IM9 cell cycle arrest at G2/M phase in a dose-dependent manner. Scriptaid triggered IM9 cell apoptosis was obviously, the mRNA levels of apoptosis-related proteins Caspase 9, Caspase 3 and PARP1 were also activated. The apoptosis-associated factors BAD, PTEN and p21 increased following treatment with different dose of Scriptaid, meanwhile, p21 promoter activity was also activated significantly. CONCLUSION: HDAC inhibitor Scriptaid can promote IM9 cell apoptosis by transcriptional activation of p21 promoter in concentration-dependent manner.